The sun emits ultraviolet (UV) radiation in the form of A rays (UVA), B rays (UVB), and C rays (UVC). The approximate wavelength band, or range, in nanometers for UVA is 400-320 nm, UVB is 320-280 nm, and UVC is below 280 nm. Due to absorption in the atmosphere's ozone layer, 99% of the ultraviolet radiation that reaches the Earth's surface is UVA. The total UVA and longer wavelengths of light at the Earth surface vastly exceeds that of UVB, with this differential increasing with increasing latitude, decreasing altitude, increasing daytime from solar noon, and temporal distance from summer solstice.
The Sun Protection Factor (SPF) is a laboratory measure of the effectiveness of sunscreen: the higher the SPF, the more protection a sunscreen offers against UVB (the ultraviolet radiation that causes sunburn).
SPF represents an endpoint indicative of sensitivity to UVB. The SPF endpoint indicates the time a person can be exposed to sunlight before developing a condition commonly known as “sunburn” with a sunscreen applied relative to the time they can be exposed without sunscreen. For example, someone who would develop the sunburn condition—or also more commonly “burn”—after 12 minutes in the sun would expect to burn after 2 hours (120 min) if protected by a sunscreen with SPF 10. In practice, the protection from a particular sunscreen depends on a variety of factors including skin type, amount and frequency of sunscreen applied, and amount of sunscreen the skin absorbs, to name a few. Sunscreen is commonly considered to be a lotion applied topically and promoted as a way of reducing sunburn.
SPF is typically measured by applying sunscreen to the skin of a volunteer and measuring how long it takes the volunteer to develop a sunburn when exposed to an artificial sunlight source. This is considered an “in vivo”, measurement of the efficacy of a sunscreen. SPF can also be measured “ex vivo”, that is, through experimentation in a controlled environment outside a living organism. Ex vivo measurements of SPF are typically performed with the help of a specially designed spectrometer, that is, a device used to measure properties of light. In this case, the actual transmittance of the sunscreen is measured, along with the degradation of the product due to being exposed to sunlight. The transmittance of the sunscreen must be measured over all wavelengths in the UVB band (290-350 nm), along with a table of how effective various wavelengths are in causing sunburn (the erythema action spectrum) and the actual intensity spectrum of sunlight, or solar irradiance spectrum. Such ex vivo measurements agree very well with in vivo measurements.
Mathematically, the SPF is calculated from measured data as
  SPF  =            ∫                        A          ⁡                      (            λ            )                          ⁢                  E          ⁡                      (            λ            )                          ⁢                  ⅆ          λ                            ∫                        A          ⁡                      (            λ            )                          ⁢                              E            ⁡                          (              λ              )                                /                      mPF            ⁡                          (              λ              )                                      ⁢                  ⅆ          λ                    where; E(λ) is the solar irradiance spectrum; A(λ) is the erythema action spectrum; and mPF(λ) is the monochromatic protection factor; and all functions of the wavelength λ. The mPF is roughly the inverse of the transmittance at a given wavelength λ, or the ratio of the ultraviolet intensities recorded at wavelength λ before and after application of a sun protecting product. More simplistically, SPF is time or dose of UVB rays to cause minimal erythema on protected skin divided by time or dose of UVB rays to cause minimal erythema on unprotected skin. Erythema is an abnormal redness of the skin caused by solar radiation, commonly termed sunburn.
SPF is an imperfect measure of skin damage because invisible damage and skin aging is also caused by the very common UVA, which does not cause reddening or pain. SPF does not measure the UVA protection of sunscreens. Conventional sunscreens do not block UVA as effectively as UVB. UVA also causes deoxyribonucleic acid (DNA) damage to certain cells deep within the skin, known as melanocytes, thereby increasing the risk of photoaging, including wrinkles or discoloration, and melanoma. Melanoma is a malignant tumor of melanocytes. Melanocytes are cells located in the bottom layer of the epidermis, or outermost layer of the skin. Although melanoma is one of the rarer types of skin cancer, it causes the majority of skin cancer related deaths.
Melanoma rates have been increasing over the years. Annual percent increases were of 2.5% between 1992 and 2001. Melanoma has disproportionately high mortality in younger age groups, such as 18 to 40 year olds, with each death resulting in a loss of almost 19 years of expected life, among the highest for adult onset cancers. Although it remains unclear how ultraviolet light causes melanoma, it is suggested that melanoma is caused by oxidative stress damage to DNA in the melanocytes caused by longer wavelengths, such as UVA.
Difficulties remain in measuring real-life UVA protection of sunscreens. Current methods to measure UVA protection include Persistent Pigment Darkening (PPD) and Immediate Pigment Darkening (IPD). PPD is the persistent darkening of the skin observed after UVA exposure whereas IPD is the transitory darkening of the skin observed after UVA exposure. Although these methods measure UVA protection, these methods are not accurate, reliable or reproducible. These methods fail to determine important parameters such as photostability of the skin with sunscreen, absorption and permeation of sunscreen, and water-resistance of the sunscreen.
Persistent Pigment Darkening (PPD) measures UVA protection by comparing results from sunscreen protected skin and unprotected skin to determine UVA-protection factors (UVA-PF). PPD's clinical significance is said to be questionable because the spectrum for PPD is not defined for wavelengths shorter than 320 nm. PPD requires high doses of UVA, which in some instances is unrealistic. In addition, the results are masked during outdoor sun exposure by other skin responses to ultraviolet radiation. Thus, it is impossible to relate the PPD protection factor directly to the degree of UVA protection to sunlight.
Immediate Pigment Darkening (IPD) concerns immediate reactions induced by UVA radiation on the skin surface. IPD is thought not to be a precise method since it is difficult to detect for all skin phototypes. Skin phototype is determined by the amount of melanin pigment in the skin. Skin phototype is based on a scale from one (pale white skin) to six (dark brown or black skin). Problems with IPD include that it does not show up on pale or fair phototypes and it is difficult to detect on dark phototypes. The IPD that develops after exposure to UVA rays does not allow for a precise measurement of UVA protection to sunlight.
There is a demand for a measure of a skin protection factor of UVA rays that is accurate, reliable, and can be used as a world-wide standard. The present invention satisfies this demand.